The present invention relates generally to damping spacers for overhead parallel conductors and particularly to a damping spacer having a variable damping characteristic that is effective to dampen aeolian vibrations in both low and high temperature environments as well as wake-induced oscillations that occur in conductor bundles.
A properly designed damping spacer for overhead conductor bundles should be capable of controlling two major forms of conductor motion, namely, an aeolian vibration and a wake-induced oscillation of one or more of the bundle conductors. respectively. aeolian vibration is a conductor motion of relatively high frequency and low amplitude; it results from smoothly flowing winds moving at velocities of 2 to 15 miles per hour. Wake-induced oscillation, on the other hand, comprises motion of a relatively low frequency and large and sometimes clashing amplitudes. This type of motion is peculiar to bundle conductors and arises from the effects of the shielding of the leeward conductor by the windward conductor. The wake-induced phenomena is discussed in U.S. Pat. Nos. 3,925,594 and 4,018,980 to Rawlins and Mohajery et al, respecively.
The design of an elastomer damping spacer for damping aeolian vibration is complicated by the effects of changes in temperature; i.e., when the temperature falls, the material of the damping elastomer tends to harden such that it is less amenable to working by the low energy of aeolian motion. For this reason the design of the spacer and the material of the damping element should provide a characteristic that is "soft" so that aeolian vibration is damped in the worst case, i.e., at the lowest anticipated cold weater condition. And for this reason such softness is ineffective as a damping mechanism when there is high energy input to the conductor under conditions of wake-induced oscillation.
In U.S. Pat. No. 4,223,176 to Hawkins, a damping spacer is shown in which damping elements are protected from the ultraviolet degradation of the sun by a hub interlock structure that is also effective to protect the damping elements from harm by wake-induced and other high energy oscillations. This latter function is acomplished by integral metal wall structures of the hub that act as stop means when conductor motion beomes excessive. However, under conditions where excessive motion of conductors is prolonged, the metal walls of the hub can become damaged due to prolonged, continuous impacting of the wall structures. Under such conditions, it would be better to have a stop mechanism that does not involve impacting of metal structures.
A damping spacer having motion limiting stop means introduces a shock into the system of the conductor bundle when the bundle experiences the phenomena of wake-induced oscillation. When a shock is applied to a distributed system, such as a conductor bundle, waves occur in the manner of the waves that radiate from the location where a stone thrown into a pond strikes the surface of the pond. The wavelengths and frequencies of these waves are not directly related to the frequency of the wake-induced oscillation, or the frequency at which stones are thrown into a pond. Generally, the frequencies of impact waves in a conductor bundle are higher than the frequency of the wake-induced oscillation. The source of the energy of the impact waves, i.e., the wind, is the same as that causing the conductor to oscillate. Hence, the energy imparted to impact waves is diverted from the energy of the oscillations induced by the wind. For this reason impacts provided by damping spacers appear as a form of damping. In addition, the impact waves that are generated, since they are of a higher frequency than wake-induced oscillations, will generally not correspond to a mode of conductor oscillation that is unstable, i.e., to a mode that tends to oscillate. Rather, the impact waves disperse through the bundle system and die out.